1. Field of the Invention
The present invention relates to a medical X-ray imaging apparatus for performing tomographic imaging along a desired tomographic plane of a subject, such as the head, body, and hands and legs of the human body.
2. Description of the Related Art
As related arts, Japanese Unexamined Patent Publication JP-A 61-22841 (1987) and Japanese Unexamined Utility Model Publication JP-U 4-48169 (1992) disclose an X-ray imaging apparatus for performing TDI (Time Delay Integration) of an image signal by changing the frequency of a charge transfer clock in accordance with the movement of an X-ray image formed on a CCD sensor while being moved.
However, in the prior art of JP-A 61-22841 and JP-U 4-48169, in case the CCD sensor causes variations in characteristics or the intensity of X-rays changes, artificial images are generated in an X-ray image, causing not only reduction in image quality and resolution but also the danger of wrong diagnosis. In addition, in the case of dental panoramic imaging, where the X-ray transmission ratio of a subject changes significantly during imaging because of the presence of the vertebra or prosthesis, the signal level of an X-ray image is changed greatly, thereby making the observation very difficult.
Japanese Unexamined Patent Publication JP-A 62-43990 (1988) discloses a method and an apparatus for X-ray imaging by scanning a charge pattern corresponding to X-ray intensity distribution in a space by using an electrometer after X-ray imaging, wherein artifacts (artificial images) are reduced by correcting the X-ray image.
However, in JP-A 62-43990, signal processing completely different from that of the TDI method is performed by using a special X-ray imaging device referred to as an X-ray converting photoconductor made of selenium. It is, therefore, difficult to directly apply the image correction method to the TDI method.
Furthermore, Japanese Unexamined Patent Publication JP-A 60-160947 (1985) discloses a panoramic X-ray imaging apparatus using an X-ray film, wherein autoexposure and central density correction are attained by controlling the tube voltage and tube current of an X-ray generator on the basis of a comparison between X-ray film feeding speed and a residual X-ray dosage having passed through a subject.
Furthermore, Japanese Unexamined Patent Publication JP-A 60-59700 (1985) discloses a panoramic X-ray imaging apparatus which detects a residual X-ray dosage having passed through a subject by using a sensor composed of a plurality of segments and feeds back the detection results to control an X-ray source, wherein autoexposure is compensated for so as to obtain a proper density at the time of starting imaging of a special site.
However, JP-A 60-160947 and JP-A 60-59700 relate to methods of auto exposure and central density correction in a panoramic X-ray imaging apparatus using an X-ray film. It is, therefore, very difficult to directly apply the methods to the TDI method.
Furthermore, Japanese Unexamined Patent Application JP-A 6-38950 (1994) discloses an X-ray imaging apparatus wherein a photodiode for monitoring X-ray dosages is formed on the bottom surface of a substrate so as to control the output of an X-ray source by real-time detection of X-rays having passed through a subject during imaging.
However, since the apparatus described in JP-A 6-38950 performs real-time detection of X-rays having passed through a subject during imaging by using the photodiode disposed on the bottom side, the response of the entire loop must be considerably quick to stabilize an X-ray dosage by feeding back the detection signal to the X-ray source. This cannot be attained easily in actual practice.